The nominate subspecies of this parakeet is 29 cm or 9 to 11 in long on average, with a 48 cm wingspan, and weighs 100 g. Females tend to be 10–20% smaller but can only be reliably sexed by DNA blood or feather testing. It has bright green upperparts. The forehead and breast are pale grey with darker scalloping and the rest of the underparts are very-light green to yellow. The remiges are dark blue, and the tail is long and tapering. The bill is orange. The call is a loud and throaty chape(-yee) or quak quaki quak-wi quarr, and screeches skveet.[2]

Domestic breeds in colors other than the natural plumage have been produced. These include birds with white, blue, and yellow in place of green. As such coloration provides less camouflage, feral birds are usually of wild-type coloration.

Myiopsitta monachus is presently the only unequivocally accepted member of the genusMyiopsitta. However, it seems that the Cliff Parakeet (see below) will eventually be recognized as a species again, as it has been on-again-off-again since it was first described in 1868.[3] It is presently included with the Monk Parakeet because there is too little up-to-date research on which an authoritative taxonomic decision could be based. The AOU for example has deferred recognizing the Cliff Parakeet as distinct "because of insufficient published data".[4]

The first three subspecies' ranges meet in the general area of Paraguay, and there they are insufficiently delimited. The distinctness and delimitation of calita and cotorra especially requires further study. Regarding the Cliff Parakeet, it appears that its altitudinal range does not overlap calita/cotorra's and that it is thus entirely—but just barely—allopatric.[6]

The Monk Parakeet is globally very common,[7] and even the rather localized Cliff Parakeet is generally common.[3] In Argentina, Brazil and Uruguay, Monk Parakeets are regarded as major agricultural pests (as noted by Charles Darwin among others). Their population explosion in South American rural areas seems to be associated with the expansion of eucalyptusforestry for paper pulp production, which offers the bird the opportunity to build protected nests in artificial forests where there is small ecological competition from other species. The Cliff Parakeet occasionally plunders maize fields but it is apparently not considered a major pest as there is no serious persecution.[3]

The Monk Parakeet is the only parrot that builds a stick nest, in a tree or on a man-made structure, rather than using a hole in a tree. This gregarious species often breeds colonially, building a single large nest with separate entrances for each pair. In the wild, the colonies can become quite large, with pairs occupying separate "apartments" in nests that can reach the size of a small automobile. These nests can attract many other tenants including birds of prey such as the Spot-winged Falconet (Spiziapteryx circumcincta), ducks such as the Yellow-billed Teal (Anas flavirostris), and even mammals. Their 5-12 white eggs hatch in about 24 days.

The Cliff Parakeet, as its name implies, rather nests in cliff crevices. This taxon rarely builds communal nests, but individual pairs still prefer to nest in close association.[3]

Unusually for a parrot, Monk Parakeet pairs occasionally have helper individuals, often a grown offspring, which assists with feeding the young (see kin selection).

The lifespan of Monk Parakeets has been given as 15–20 years[8] or as much as 25–30 years;[9] the former might refer to average lifespans in captivity and/or in the wild, while the latter is in the range of maximum lifespans recorded for parakeets.

Monk Parakeets are highly intelligent, social birds. Those kept as pets routinely develop large vocabularies. They are able to learn scores of words and phrases.[10] Due to this early speaking ability, the Quaker Parrot is overtaking the cockatiel as the favorite bird to teach to talk. Another asset is that this bird has a much more reasonable life span and price than the African Greys or the Yellow Naped Amazons.

In areas where they have been introduced, some fear that they will harm crops and native species. Evidence of harm caused by feral colonies is disputed, and many people oppose killing this charismatic bird. However, there have been local bans and eradication programs in some areas of the USA. Outside the USA, introduced populations do not appear to raise similar controversy, presumably because of smaller numbers of birds, or because their settlement in urban areas does not pose a threat to agricultural production. The UK appears to have changed its view on its feral populations and the Department for the Environment, Food and Rural Affairs is planning to remove Monk Parakeets from the wild,[11] as it believes that they threaten local wildlife and crops.

It was found that feral populations are often descended from very small founder populations. Being as social and intelligent as they are, Monk Parakeets will develop some cultural traditions, namely vocal dialects that differ between groups. In populations descended from a large number of birds, a range of "dialects" will exist. If the founder population is small however, a process similar to genetic drift may occur if prominent founders vocalize in an unusual "dialect", with this particular way of vocalizing becoming established in the resulting feral colony. For example, no fewer than three different "dialects" occur among the feral Monk Parrots of the Milford, Connecticut, metropolitan area.[12]

The species has in recent years expanded its range in Brazil, where there is now a self-sustaining population in the downtown area of Rio de Janeiro. Since this population occurs far from the bird's original range in Brazil - it was only found in the far south and southwest - it is most probably a consequence of escapees from the pet trade. In Rio de Janeiro, the bird can be easily seen at the Aterro do Flamengo gardens - where it nests on palm trees and feeds on their fruit; the Rio birds seem to favor nesting amid the leaves of coconut palm trees - as well as in the vicinity of the neighboring domestic flight terminal, the Santos Dumont Airport and in the gardens of Quinta da Boa Vista, where communal nests of roughly one meter in diameter have been seem.[13] In Santa Catarina State, probable escapees have been reported on occasion since quite some time, and a feral population seems to have established itself in Florianópolis in the early 2000s (decade) when birds were observed feeding right next to the highway in the Rio Vermelho-Vargem Grande area.[5]

Considerable numbers of Monk Parakeet were imported to the United States in the late 1960s as pets. Many escaped or were intentionally released, and populations were allowed to proliferate. By the early 1970s, M. monachus was established in seven states, and by 1995 it had spread to eight more. There are now thought to be approximately 100,000 in Florida alone.

In addition, they have also found a home in Brooklyn, New York, after an accidental release decades ago of what appears to have been black-market birds[14] within Green-Wood Cemetery. The grounds crew initially tried to destroy the unsightly nests at the entrance gate, but no longer do so because the presence of the parrots has reduced the number of pigeons nesting within it. The management's decision was based on a comparative chemical analysis of pigeon feces (which destroy brownstone structures) and Monk Parakeet feces (which have no ill effect). Oddly then, the Monk Parakeets are in effect preserving this historic structure. Brooklyn College has a Monk Parakeet as an "unofficial" mascot in reference to the colony of the species that lives in its campus grounds. It is featured on the masthead of the student magazine. They have also made their homes in the lamp posts in Pelham Bay Park in the Bronx. Most of these Quaker Parrot populations can be traced to shipments of captured Quaker Parrots from Argentina.[15]

Because of Quaker Parrots' listing as an agricultural pest, California, Georgia, Kansas, Kentucky, Hawaii, New Jersey, Pennsylvania, Tennessee, and Wyoming outlaw sale and ownership of a monk parakeet. In Connecticut, one can own a Quaker Parrot, but cannot sell or breed them. In New York and Virginia, it is possible to own a Quaker Parrot with banding and registration. In Ohio, owning a Quaker Parrot is legal as long as the bird's wings are clipped so that it cannot fly.[15]

The Monk Parakeet was first recorded in Mexico City in 1999.[19] There are also records for seven other locations, including the cities of Puebla, Morelia, Celaya, Oaxaca, Tuxtla Gutiérrez, Hermosillo, and the mouth of the Loreto River in Baja California Sur.

Nesting populations are known in Mexico City and Oaxaca. A small but growing population has also established in the southern part of the city of Puebla, Puebla, in the surroundings of the city's aviary, which they are known to visit frequently, and where they can often be seen clinging to the outer side of its mesh walls. No studies have been made to assess the impact they might have on the relict populations of Green Parakeet that live in the same area and other well wooded zones of the city.

Following the ban on the trade of native parrot species, local traditional bird sellers have now switched to the monk parakeet as their staple parrot, and that might have increased the number of escapees. Sometimes the head and breast feathers of Monk parakeets are dyed yellow to deceive uninformed buyers, mimicking the endangered Yellow-headed Amazon. The presence of this species in 7 geographically distant and independent locations in Mexico indicates that the source of these individuals is most likely the pet trade.[19]